Travelling wave devices



April 2, 1963 J. M. osEPcHuK TRAVELLING WAVE DEVICES Filed Feb. 10, 1960 2 Sheets-Sheet l 2 6 0 m J W A w WM R w 7 0 8 9 m A N B w m A@ W m R 6 3 w a P\ w w 4% k m .A W m REGION ONE f REGION TWO l IN VE/VTOR JOHN M. OSEPGHUK ATTORNEY United States Patent Ofifice tasters ?atented Apr. 2, 19%3 3,ll34,279 TRAVELLING WAVE DEVitIEEa John M. Osepehulr, Lexington, Mass, assignor to Raytheon Company, Lexington, Mass, a corporation of Delaware Filed Feb. 10, End, Ser. No. 7,821 6 Claims. (1. 315-395) This invention relates to travelling wave electron discharge devices and, more particularly, to methods and means of construction of an M-type backward wave oscillator having novel means to insure proper electron bearn collection.

in travelling wave electron discharge devices, electrical waves travelling along a slow wave propagation structure in a tube interact with the field from electrons moving along paths adjacent said structure to produce amplification of the travelling wave. The longitudinal component of the field of the wave interacts with the field of the electrons travelling along in approximate synchronism with it. Some electrons are accelerated and others are decelerated resulting in a progressive arrangement of electrons in phase with respect to the waves. This process of mutual interaction continues along the length of the structure with the result that D.C. energy is given up by the electron beam to the wave as radio frequency energy and, thus, the wave is amplified.

In a travelling wave tube the slow wave propagation structure may be considered to consist of a number of cells in line. Since such a structure is periodic, the fields induced in adjacent cells by the propagated wave, have the same spatial distribution but differ uniformly in phase. The electron beam field interacts with the fields in the cells and this interaction is maximum when the velocity of the electrons is such that their fields encounter the fields in successive cells in the same phase. In other words, maximum interaction occurs when the electron velocity coincides with the phase velocity of RF waves propagating either in the same or in the opposite direction to the direction of RF power flow. These RF waves are called direct and reverse, respectively, depending on the direction of RF power flow. Consequently, when the beam velocity is equal to the velocity of the reverse wave, the amplitude of the fields in the cells increases towards the origin of the beam. One type of travelling wave tube, sometimes called a backward wave oscillator, produces radio frequency oscillations by the interaction of an electron beam travelling at substantially the same velocity as the reverse wave.

In one type of backward wave oscillator, sometimes called a type M carcinotron, the electron beam moves in crossed electric and magnetic fields in the interelectrode space between a wave propagation structure called the line, and an elongated electrode parallel thereto, called the sole. The average translational velocity of an electron in the beam is equal to the ratio of the electric field E to the magnetic field B. The variation of the oscillator frequency with line-to-sole voltage is determined by the dispersion characteristics of the beam and the average electron velocity. Therefore, frequency depends on both the electrical field and the magnetic field.

As the electron beam gives up energy to the radio frequency wave in the line, it loses potential energ however, it maintains the same translational velocity. Since the beam electrons lose potential energy, they must move through substantially higher equi-potential lines of the electrical field E between the line and the sole. Consequently, electrons will move either towards the'line or towards the sole, depending on the direction of the field E, and eventually these electrons will be captured by a collector electrode. In some applications the collector is at the same potential as the line and in others it is at the same potential as the sole. If the beam is projected into the interelectrode space between the line and the sole along an equi-potential line V and the potential between the line and the sole is V, the electrical efficiency of the system, 1 is given by the following equation which assumes that electrons move over ideal linear trajectories: '71:1--V0/V The efficiency 1; is further modified by space charge effects and the fact that some of the electrons are captured by the line or the sole, depending on which is at the higher potential. It is generally preferred to ground the line and the collector and to vary sole potential to change E and, thereby, tune the oscillator.

Heretofore, the problem of preventing electron leakage past the collector has been met by making the distance between the collector and the sole approximately one half the distance between the line and the sole, as shown in FIG. 1. Such a design insures that substantially all of the beam electrons will be collected by the collector and substantially none will leak past the collector. Since the separation between the line and the sole is, in most ap plications, very small (between .100 and .010 inch), it is, obviously, difficult to construct such a device with such small separation between collector and sole. Furthermore, by placing the anode and the sole so very close, arcing is likely to occur because the potential difference between the anode and sole is usually very large (many thousands of volts). Such arcing is obviously undesirable since its immediate and ultimate effect is to introduce considerable noise in the output of the device. Therefore, it is an object of the present invention to provide a backward wave oscillator having none of the above-mentioned limitations of prior devices.

It is another object'of the present invention to provide an electron collector in an M-type backward wave oscillator for collecting a substantial par-t of the electron beam therein.

It is another object to provide such a collector structure wherein equal electrical fields are maintained between the collector and the soleand between the line and the sole.

It is another object to provide a backward wave oscillator in which beam dispersion is substantially greater between the collector and the sole than between the line and the sole.

it is another object to provide an M-type backward wave oscillator in which the magnetic field in the interelectrode space between the collector and the sole is substantially less than the magnetic field between the line and the sole.

It is another object to provide a backward wave oscillator tunable by varying sole potential while at the same time maintaining the electric field adjacent to the sole uniform throughout the length of the sole.

it is a feature of the present invention to provide a travelling Wave type electron discharge device including an elongated structure at one potential for propagating a slow wave disposed opposite a similarly elongated conductor at another potential so as to form a substantially uniform electric field in the space therebetween, to provide electron emitting means at one end of the space and electron collector means at the other end of the space, to dispose said device in a substantially uniform magnetic field and to magnetically shield a part of said space in the vicinity of said electron collector.

It is another feature of the present invention to provide a backward Wave oscillator device including a slow wave propagating structure, a collector electrode, and a sole electrode forming an interelectrode space therebetween and an electron gun projecting electrons into said interelectrode space with means in the vicinity of said collector electrode for magnetically shielding electrons in that vicinity to, thereby, insure collection of electrons by the collector and to substantially diminish flow of electronspast the collector.

It is another feature to electrically couple said electron collector to the end of said wave propagating structure thereby forming one end of the interelectrode space and to dispose magnetic materials thereabout to shield that one end from the magnetic field.

It is another feature to dispose said device in the magnetic field so that the electric field, the magnetic field and the general direction of motion of electrons within the interelectrode space are mutually perpendicular and to ob-' tain an RF output from the device at the end of the wave propagating structure nearest the electron gun.

Other features and; objects of the present invention will be more apparent from the following specific description of the prior art and embodiments of the present invention takenin conj-unction with the drawings in which:

FIG. 1 is'a symbolic diagram showing oneprior method for insuring the collection of electrons by a collector electrode in an M-type backward wave oscillator;

Z FIG. 2 is a symbolic diagram of an M -type backward wave oscillator incorporating features of the present invention; and H v a FIGS. 3 and 4 are cross sectional views of one embodiment of the present invention featuring a circular interelectrode space and suitable magnetic materials disposed about one end thereof tomagnetically shield the space in the vicinity of a collector electrode. p

Turning first to FIG. 1 there isshown symbolically a typicalM-type carcinotron such as employed in the past demonstrating one prior method for insuring that a substantial partof the electron beam will flow, to the collector electrode and not past the collector. As shown in FIG. 1 electrodes are disposed within an envelope 1 and these include a cathode 2, a slowwavestructure'or delay line 3, with an accelerating electrode 4 at one end, and a collector electrode 5 at the other end. The accelerating electrode 4, the V V are'fre quently coupled'to'gether and substantially in line as shown in FIG. 1. Another electrode,- 6, called the sole 7 electrode having an elongated shape is disposed within the envelope 1 opposite structure 3 and collector. electrode 5 tor-form interelectrode space 7. It is convenient to place structure3 and electrode 5 at ground potential and to place sole electrode 6 substantially. below groundpotential, for example, at about minus 7 kv., by coupling to potential source 8 thereby. creating electric field E in region one, runningfrom the slow'wave structure 3 to the sole 6. and electric field E inregion. two, running from collector 5 to sole 6.- Cathode 2. is preferably placed at a potential higher than electrode 6, for example, minus 5 kv., by coupling to potential source 9'. Electrons in the beam. 10, emitted from cathode intointerelectrode space 7. 'Ihisis accomplished by coupling cathode 2 and electrode 4 to potential source 10 so that accelerator electrode 4 is. at the highest potential.

' In operation, a substantially uniform magnetic field is applied perpendicular to the electricfield and is represented in FIG.. 1 by a circle with a dot in its center and denoted B. Consequently, electrons ernittedfrom cathode 2 are first accelerated towards electrode 4 and then into space 7, moving down thelength of the space at a velocity, established by the electric field. E and the magnetic field B. The general movement of the electrons downfthe length of mean 10, the electronsproceeding down the lengthof the space and dispersing near its end for collection by the collector electrode 5.

structure 3 and the collector electrode 5 2 are accelerated for entry space 7 is substantially as shown by As already mentioned in'the introduction, the beam slow wave structure 3" and as the beam gives up energy it will maintain their general translational velocity down the length of space 7. Since the beam electrons lose energy they must move through substantially higher potential lines in the electrical field between the line and the sole electrode, consequently, the electrons will move toward line 3. Part of the beam fore, is collected by the delay line, but generally there is always a substantial part of the beam entering the collector region. In order to insure almost complete collection of this latter part of the beam, it has been the practice in the past todispose col-lector electrode 5 closer to the sole electrode 6 than the structure 3. In fact, it is desirable that the space denoted d between collector electrode 51 and sole electrode '6 be approximately one half the spacing d between structure 3 and sole 6. The purpose of this is apparent when one considers the relationship between the potential difference V across an interelectrode space required to insure complete beam collection by an electrode, and the dimensions d. It canbe shown that this relationship is given by the following:

charge to mass ratio arcing is likely to occur therebetween resulting in numer-. ous undesired effects.

Turning next to FIG. 2 there is shown a symbolic representation incorporating features of the present inof arcing are substantially eliminated. Thestructure shown in FIG. 2 is similar to the structure shown in FIG. 1 and includes an envelope 1-1 enclosing a slow wave propagation structure .12, accelerating electrode 13, collector electrode 14, sole electrode 15fand cathode 16 forming interelectrode space 17 with an RF output taken from oneend of structure 12, denoted 18. In operation, electrode 14 and structure 12, may be electrically coupled together and placed at, for example, ground potential while sole electrode 15 is placed at, for example, minus 7 k.v. by coupling to source 19, cathode 116 is placed at minus 5 k.v. by coupling to source 20 and accelerating electrode 13 is placed at minus 3 k.v. by coupling to source 20 as shown in the figure. The device is placed in a magnetic field which is perpendicular to the electric field also perpendicular to the general motion of electrons within the space represented by beam 21. Electrons emitted from cathode 16 are accelerated by accelerating electrode 13 andproceed down the length of space 17 moving to higher and higher equi-potential lines as energy is given up from the electron beam to the wave in structure 1 2. Magnetic shielding pieces 22 and 23 are disposed substantially contiguously with collector electrode 14 and one'end of sole electrode 15, respectively, as shown in the figure. These magnetic shielding pieces may be, for example, flat in shape and orientated with their smallest dimension perpendicular to the applied magnetic field so as to afford magnetic shielding in region two of interelectrode space 17 formed by the collector electrode 14 and one end of sole electrode 15. Consequently, the magnetic field in region two of space 17 is substantially less than the magnetic field in region one. If the magnetic field in region one is denoted B and the magnetic field in region two is denoted B then the expression for the voltage difierential V required to insure under eflicient interaction, there magnitude of the transverse created within space 17 and complete collection of the electron beam by the collector electrode '14 is set forth as follows:

Obviously, if d and d are equal and of sufiicient magnitude to insure no arcing, and B is fixed at a suitable value for maintaining the ration E/B to obtain a desired electron velocity and oscillator frequency, B may be varied to provide a suitable value of V This is accomplished by employing any number of properly shaped and orientated magnetic shielding pieces such as 22 and 23 to shield region two of the space 17. The shape of the pieces 22 and 23, the number of such pieces and their orientations may all be varied to achieve the proper amount of shielding so as to provide a suitable magnetic field B in region two of the space. It has been found advisable to maintain the dimension 1 of magnetic pieces 22 and 23 greater than 1rd It is also preferable that the ratio of B /B be less than one third to insure substantially complete beam collection by electrode 14.

Turning next to FIG. 3 and FIG. 4 there are shown cross sectional views of an M-type backward wave oscillator having a circular interelectrode space 24 and incorporating features of the present invention. 'FIG. 3 shows a top sectional view BB in which appears a ring shaped sole 25 concentric with a ring shaped slow wave propagating structure 26. Structure 26 might, for example, be comprised of two sets of fingers 27 and 28 interlaced and held in place by fastening rings 27a and 28a, from which the fingers project, to conductive cylinder 29; thereby, forming a tortuous path for slow wave propagation. Such a structure is described in considerable detail in Patent No. 2,890,372, issued June 5, 1959, to E. C. Dench, and entitled Travelling Wave Amplifiers.

Sole 25 might, for example, have a channel-like cross sectional shape as shown in FIG. 4. Sole 25 and slow wave structure 26 are disposed concentric with each other so as to provide interelectrode space 24 therebetween. A cathode 30 and accelerator electrode 31 are inserted between the open ends of the ring formed by sole 25- and encloses a heater element 32. Electrical leads run from cathode 3t electrode 31, element 32 and sole 25 to conductors 33, 34, 35 and 36, respectively, at the center of the device.

Structure 26 also forms an open ring, the opening therein being partially opposite the opening in the ring formed by sole 25 and collector electrode 37 is inserted in this opening. Electrode 37 may be electrically coupled to the structure to form one end of interelectrode space 24 referred to as region two in the straight line device of FIG. 2. t i

In operation, a magnetic field from magnet 3-8 is applied to interelectrode space 24 so that electrons emitted from cathode 30 enter the space and move in a substantially circular path around the space in a counterclockwise direction, as shown by the arrow on beam 39, giving up energy to the RF wave propagated in structure 26. As the electrons move through space 24 as a beam and approach collector electrode 37, the beam disperses, electrons therein moving to higher potential lines and, thus, moving closer to the fingers of structure 26. By proper orientation of the magnetic shielding pieces 40 and 41 fixed to sole 25 and collector 37, respectively, and adjustment of the strength of magnet 38, substantially complete collection of the beam by collector electrode 37 can be achieved while at the same time maintaining a desired radio frequency output from the device. Such an output might, for example, be taken from finger 42 by coupling to coaxial conductor 43 as shown.

In FIG. 4 there is shown cross sectional view AA taken through the magnetic shielding pieces 44} and 41 and the center of the device. In this view a suitable shape for magnet 38 is shown. The field from the poles of magnet 38 is conducted through magnetically permeable rings 44 and 45. Consequently, the magnetic flux lines travel from ring 44 to 45 through the interelectrode space 26.

The magnetic shielding pieces 40' and 41 are preferably located with a major dimension parallel to this magnetic field. One piece might, for example, be mounted inside the ring of sole 25 as shown and another might, for example, be located between collector electrode 37 and conductive ring 29 as shown in FIG. 4. The shape and orientation of parts forming structure 26 are more clearly shown in FIG. 4, while the location of the fingers are more apparent in FIG. 3. Rings 27a and 28a supporting these fingers are suitably fastened to conductive ring 29 to form the complete slow wave structure 26. The fastening may, for example, be accomplished as shown in the referenced patent.

Upper and lower cover plates 46 and 47 are provided for enclosing the structure 26, interelectrode space 24, sole 25 and cathode 30 so as to form a vacuum seal with conductive ring 29. Electrical conductors 33, 34, 35 and B6 are brought out through terminals 48, 49, 50 and 51 in a suitable electrical feed through 52 which forms a vacuum seal with upper plate 46 and suitable potentials are applied to these terminals for heating cathode 30, accelerating electrons emitted therefrom and providing a uniform electrical field within space 24 while the other parts of the device including upper and lower plates 46 and 47, collector electrode 37 and interdigital delay line 26 are placed at ground potential.

While the present invention is described herein with reference to a symbolic representation of an M-type backward wave oscillator and with respect to a more specific embodiment of such an oscillator including a circular interelectrode space for insuring substantially complete beam collection by a collector electrode, it is to be understood that the spirit and scope of the present invention may be incorporated in other type travelling wave devices to achieve substantially the same or similar results without deviating from said spirit or scope as expressed in the following claims.

What is claimed is:

1. An electron discharge device comprising slow wave propagating means at one potential, an elongated electrode at another potential disposed with relation to said propagating means forming an elongated space and bounding an electric field within said space, electron emitting means at one end of said space, electron collecting means at the other end of said space and means producing a transverse magnetic field in said space substantially uniform throughout and means at the end of said space for substantially altering said magnetic field in the vicinity of said collecting means to thereby insure collection of a substantial part of said beam by said collecting means.

2. An electron discharge device comprising slow wave propagating means at one potential, an elongated conductor at another potential disposed with respect to said propagating means to form an elongated interelectrode space having a substantially uniform electric field Within, electron emitting means at one end of said space, electron collecting means at the other end of said space and means producing a uniform magnetic field in said space substantially perpendicular to said uniform electric field and also perpendicular to the general movement of electrons within said space and means at the end of said space for substantially altering said magnetic field in the vicinity of said collecting means to thereby insure collection of a substantial part of said beam by said collecting means.

3. An electron discharge device comprising slow wave propagating means at one potential, an elongated conductor at another potential disposed with respect to said propagating means to form an elongated interelectrode space having a substantially uniform electric field within, electron emitting means at one end of said space, electron collecting means at the other end of said space, means producing a magnetic field in said space substantially perpendicular to said electric field and also perpendicular tothe general movement of electrons within said-fspace' and means .for'magnetically shielding apart of said space in the vicinity of said electron collecting mean'sto there by insure collection of a snbstantial part of said beam by saidcollec ting means. a

4.'An' electron discharge device comprising means for propagating an electrical Wave in-a delay structure, a

conductor displaced from said structure forming an inter electrode space t lierebetv'veei'l, means producing substantially' crossed electric and magnetic fields within said space, means emitting electrons at one'end' of said space, means collecting electrons at the other end of'said space andmeans at said other'end for reducing the" magnitude:

of said magnetic field in the vicinity of said electron collector to thereby insure collection of a substantial part of said 'beam by said collecting means.

5. electrical discharge device comprising slowwave' propagating means" andconducting means having substantially elongated shapes and disposed with respect to each other to form an elongated interelect'rod'e space, means emitting a beamofelectrons at 'one end of said space,-'

means contiguous vvitlf said propa atin means disposed at the other end of said space for collecting electrons, means prodficing crossed electric and magnetic fields in said space and means disposed about said other end of' saidspacefor magnetically shielding" said space in the 8? vicinity of said collecting means to' thereby insure. collection of a substantial; partof said beam by said collect: ing means.

6. An -M-type backward wave oscillator including a r 5 slow wave structure, a sole electrode forming an elongated "lectioxiof a substantial part of said beam by said collecting means.

References Cited in the file of this patent UNITED STATES PATENTS 2,607,904 Lerbs c Aug. 19, 1952 2,829,299 Beck Apr. 1, 1958 2,844,754 Cioffi July 22, 1958 2,853,641 Webber Sept. 23, 1958 2,871,395 Cioffi Jan. 27, 1959 2,880,355 Epsztein Mar. 3 1, 1959 3,003,119 Favre Oct. 3, 1961 

1. AN ELECTRON DISCHARGE DEVICE COMPRISING SLOW WAVE PROPAGATING MEANS AT ONE POTENTIAL, AN ELONGATED ELECTRODE AT ANOTHER POTENTIAL DISPOSED WITH RELATION TO SAID PROPAGATING MEANS FORMING AN ELONGATED SPACE AND BOUNDING AN ELECTRIC FIELD WITHIN SAID SPACE, ELECTRON EMITTING MEANS AT ONE END OF SAID SPACE, ELECTRON COLLECTING MEANS AT THE OTHER END OF SAID SPACE AND MEANS PRODUCING A TRANSVERSE MAGNETIC FIELD IN SAID SPACE SUBSTANTIALLY UNIFORM THROUGHOUT AND MEANS AT THE END OF SAID SPACE FOR SUBSTANTIALLY ALTERING SAID MAGNETIC FIELD IN THE VICINITY OF SAID COLLECTING MEANS TO THEREBY INSURE COLLECTION OF A SUBSTANTIAL PART OF SAID BEAM BY SAID COLLECTING MEANS. 